Nonlinear relaxation behavior and competing aging mechanisms in GAP-based propellants under thermal aging

Glycidyl azide polymer (GAP)-based propellants, known for their high energy efficiency, exhibit unique nonlinear variations in viscoelastic behavior during thermal aging, which is distinct from the monotonic trends observed in traditional propellants. This paper investigates the relaxation behavior of GAP-based propellants subjected to thermal aging at 60 °C. Nuclear Magnetic Resonance and high-performance liquid chromatography analyses are conducted to reveal the underlying mechanisms driving the nonlinear relaxation response. The aging process is classified into three distinct stages: an initial phase dominated by post-curing reactions, followed by competing effects from crosslink network scission, and plasticizer degradation. These competing mechanisms affect the relaxation through microscopic changes in free volume, resulting in complex viscoelastic responses. A predictive model is developed for the relaxation modulus to take into account of these aging mechanisms, with capability to capture the nonlinear fluctuations in the aging shift factor. The proposed model provides accurate predictions of relaxation behavior during thermal aging, including the long-term performance of GAP-based propellants.